1. Field of the Invention
This invention relates to a light scanning optical system and an optical scanning apparatus using an oblique incidence optical system and an image forming apparatus using the same, and particularly to an apparatus such as a laser beam printer or a digital copier which well corrects the uniformity of the absolute value of the peak intensity in the light intensity distribution of a spot on a surface to be scanned and is thereby suitable for high-speed and highly definite image formation.
2. Related Background Art
A light scanning optical system (and an optical scanning apparatus) heretofore used in a laser beam printer or a digital copier or the like has a semiconductor laser as light source means, a first optical system for imaging a beam emitted from the light source means as a linear beam long in the main scanning direction near the deflecting surface of a light deflector, and a second optical system for imaging the beam reflected and deflected (deflected and scanned) by the light deflector on a surface to be scanned, and bringing the deflecting surface of the light deflector and the surface to be scanned into a substantially conjugate relation in the sub-scanning cross-section.
The second optical system of such a light scanning optical system is an anamorphic imaging optical system called an fθ lens, and has heretofore been constructed by the use of a plurality of expensive glass lenses and has been difficult to downsize.
In recent years, however, with the progress of the molding technique for plastics and the working technique for mold, it has become possible and has also been put into practical use to construct the imaging optical system (fθ lens) comprised of a plurality of expensive glass lenses by the use of an inexpensive plastic lens.
However, it is still insufficient for the downsizing of the apparatus to simply replace the glass lenses with the plastic lens. Further, the same power as the power of the glass lenses is required of the plastic lens and therefore, the deterioration of imaging performance by the fluctuation of the environment when use is made of plastics inferior in environmental characteristic also poses a problem.
So, a light scanning optical system of a post-objective type having a converging lens disposed in the optical path between the light source and the light deflector before the beam is reflected and deflected by the light deflector is known as realizing a compact and low-cost apparatus. Particularly, when the power of the fθ lens is set to a small level in order to make the environmental characteristic good, the fθ characteristic becomes under-corrected and therefore, it is often practised to make the deflecting surface of the light deflector into a convex shape. In this case, from the viewpoint of the symmetry of aberration correction, design is made such that the beam from the light source enters the light deflector at a predetermined angle from an oblique direction with respect to the deflecting surface of the light deflector, and it is often the case that the beam is reflected and deflected bisymmetrically with respect to the incident beam (with respect to the normal to the deflecting surface at the incidence point).
Also, in recent years, higher-speed and more highly definite performance has been required of apparatuses such as a laser beam printer and a digital copier.
To achieve the higher speed of the apparatus, it is necessary to rotate the light deflector at a high speed. Also, to achieve higher definition, it is necessary to make an imaged spot small in diameter and therefore, the width of the beam must be made great and accordingly, the light deflector itself for reflecting and deflecting the beam must also be made large.
To make the higher speed and higher definition compatible, the light deflector must be rotated at a high speed and be made large, but actually it is very difficult to satisfy the two at a time.
So, as a solution of such a problem, various light scanning optical systems using the so-called overfield scanning method designed such that the beam width in the main scanning direction of the beam incident on the deflecting surface of the light deflector enters (irradiates) astride the plurality of deflecting surfaces of the light deflector have been proposed and also put into practical use.
In the overfield scanning method, there is the characteristic that the size of the imaged spot in the main scanning direction is varied by the scanning angle and therefore, in order to minimize the influence thereof, it is desirable to make such design that as in the prior art, the beam incident on the light deflector is not made to enter from the main scanning cross-section, but the beam emitted from the light source is incident on the light deflector at a predetermined angle from an oblique direction with respect to the deflecting surface of the light deflector, and the beam is reflected and deflected bisymmetrically with respect to the incident beam (with respect to the normal to the deflecting surface at the incident point).
Further, the use of a multibeam laser source having a plurality of light emitting portions as light source means in the above-described light scanning optical system becomes advantageous for further higher speed.
As another means for achieving a higher speed, there is known, for example, a light scanning optical system called a twice incidence scanning method of causing a beam emitted from a light source to be incident on a first deflecting surface of a light deflector, causing the beam reflected and deflected by this first deflecting surface to be again incident on a second deflecting surface differing from the first deflecting surface of the light deflector through a transmitting optical system, and effecting deflection and scanning.
This twice incidence scanning method includes a method called a double angle scanning method, and a method called a surface following method in which a beam entering a second deflecting surface of a light deflector moves following the movement of the deflecting surface, but in both of the two methods, it is difficult to dispose a transmitting optical system in the deflecting surface and therefore, there is adopted a construction designed such that the beam emitted from the light source is incident on the light deflector at a predetermined angle from an oblique direction with respect to the deflecting surface of the light deflector, and the transmitting optical system is disposed while being deviated in the sub-scanning direction with respect to the deflecting surface in the main scanning direction.
However, in the above-described prior-art light scanning optical system, optical scanning apparatus and image forming apparatus of such a construction that the beam emitted from the light source is caused to be incident on the light deflector at a predetermined angle from an oblique direction with respect to the deflecting surface of the light deflector, if as shown in FIG. 16 of the accompanying drawings, a beam (light beam) having a coordinate system of the main scanning direction y, the sub-scanning direction z and the optical axis direction x is incident on the deflecting surface 96a of a polygon mirror 96 at a predetermined angle with respect to the sub-scanning direction, the beam reflected and deflected there does not scan in a plane but scans on a conical plane and therefore, the coordinate system of the beam rotates. At this time, the coordinate system of the beam L0 at the center of scanning does not rotate, but yet beams L1 and L2 at the opposite ends of the scanning area have their coordinate systems rotated in opposite directions as shown in FIG. 17 of the accompanying drawings.
It is well known that there is the problem that even if the coordinate system at the opposite ends of this scanning area tries to image the rotated beam on a surface to be scanned by an anamorphic imaging optical system, the beam spot shape (hereinafter simply referred to also as the “spot shape”) is destroyed and is not converged at a point, and good imaging cannot be obtained on the surface to be scanned.
To this problem, in the conventional light scanning optical system of a construction in which the beam emitted from the light source is caused to be incident on the light deflector at a predetermined angle from an oblique direction with respect to the deflecting surface of the light deflector, a scanning lens or a correcting lens is disposed eccentrically in the sub-scanning direction or only a certain surface of a lens is made eccentric in the sub-scanning direction, whereby it is possible to improve the spot shape on the surface to be scanned to a certain degree.
Heretofore, when improving the spot shape, the spot shape has been improved by evaluating the degree of the expause of a spot diagram, or the shape or the like of the equal altitude curve of the beam spot.
Actually, however, it is insufficient for obtaining a good image output to simply correct the spot shape well. Specifically, it is difficult to obtain a good image output unless the uniformity of the absolute value of the peak intensity in the light intensity distribution of the spot on the surface to be scanned is corrected well. This is because even if the shape of the spot is corrected a little well, if the absolute value of the peak intensity is low, it becomes impossible to form a good electrostatic latent image on the surface of a photosensitive drum which is the surface to be scanned.
Heretofore, there has been the problem that the uniformity of the absolute value of the peak intensity in the light intensity distribution of the spot on the surface to be scanned is not taken into account and as the result, the output image is deteriorated.
For example, Japanese Patent Application Laid-Open No. 6-18800 discloses a light scanning optical system of a post-objective type designed such that a beam emitted from a light source is incident on a light deflector at a predetermined angle from an oblique direction with respect to the deflecting surface of the light deflector, and in which the shape of the deflecting surface in the main scanning cross-section is made into an elliptical shape and the entrance surface and exit surface of a correcting lens are made into a shape in which they are displaced in the sub-scanning direction and the spot shape is corrected to a certain degree. In this example, however, the destruction of the spot shape has its spot diagram only visually evaluated and further, the correction of the shape of the spot is still insufficient, and there has been the problem that the uniformity of the absolute value of the peak intensity in the light intensity distribution of the spot on the surface to be scanned is not corrected and it is difficult to obtain a highly definite image output.
Japanese Patent Application Laid-Open No. 7-27991 discloses a light scanning optical system of the post-objective type designed such that a beam emitted from a light source is incident on a light deflector at a predetermined angle from an oblique direction with respect to the deflecting surface of the light deflector, and in which the shape of the deflecting surface in the main scanning cross-section is made into an elliptical shape and the entrance surface of a correcting lens is made into a shape in which it is displaced in the sub-scanning direction, and the spot shape is corrected to a certain degree. In this example, however, the curvature of image field in the main scanning direction by a light component widened in the sub-scanning direction of the beam (beam light) and the curvature of image field in the sub-scanning direction by a light component widened in the main scanning direction of the beam are only evaluated as the degree of the collapse of the spot shape, and this evaluating method can evaluate only the curvature of image field by a light component widened in a cross-section of a particular direction, and the curvature of image field, etc. of light component widened in the other cross-sections are not taken into account. Further, the correction of the curvature of image field in the main scanning direction is insufficient, and there has been the problem that the absolute value of the peak intensity in the light intensity distribution of the spot on the off-axis surface to be scanned is low relative to that on the axis and it is difficult to obtain a highly definite image output.
Japanese Patent Application Laid-Open No. 10-20230 discloses a light scanning optical system of the so-called surface following type in which a beam emitted from a light source is incident on a first deflecting surface of a light deflector, and the beam deflected by the first deflecting surface is again incident on a second deflecting surface of the light deflector differing from the first deflecting surface through a transmitting optical system, and the reincident light follows with the movement of the second deflecting surface, and in which a scanning lens is eccentrically disposed in the sub-scanning direction or the scanning lens is curved in the sub-scanning direction and the shape of a spot is corrected to a certain degree. In this example, however, the shape of the equal altitude curve of the beam spot is merely corrected well as the degree of the collapse of the shape of the spot. To obtain a good image output as previously described, it is insufficient to simply correct the shape of the spot well, and the uniformity of the absolute value of the peak intensity in the light intensity distribution of the spot on the surface to be scanned must be corrected well, and this point is not at all taken into account, and this has led to the problem that it is difficult to obtain a good image output.
Japanese Patent Application Laid-Open No. 11-242179 discloses a post-objective type light scanning optical system designed such that a beam emitted from a light source is incident on a light deflector at a predetermined angle from an oblique direction with respect to the deflecting surface of the light deflector, and having a construction in which a scanning lens is shifted in the sub-scanning direction. In this example, however, only the curvature of scanning lines and fθ characteristic and curvature of image field are evaluated, and the purpose of shifting the scanning lens in the sub-scanning direction takes only the curvature of scanning lines into account, and no consideration is taken about the collapse of the shape of the spot which greatly affects an output image, and this has led to the problem that it is difficult to obtain a good image output.